1. Field of the Invention
This invention relates to a process for the selective adsorption separation of mixed aromatic hydrocarbon isomers having 8 carbon atoms (hereinafter "C.sub.8 aromatic isomers") by a zeolite adsorbent.
For simplicity in the following explanation, para-xylene, ortho-xylene, meta-xylene, ethylbenzene and a desorbent are denoted by the symbols "PX", "OX", "MX", "EB" and "DS", respectively.
Also, the selectivity of a system comprising a zeolite adsorbent and a solution for one substance A over another substance B is denoted by the formula; ##EQU1## wherein X.sub.A represents a concentration of substance A in the zeolite adsorbent phase; X.sub.B represents a concentration of substance B in the zeolite adsorbent phase; Y.sub.A represents a concentration of substance A in the external solution adjacent to the zeolite adsorbent; and Y.sub.B represents a concentration of substance B in the external solution adjacent to the zeolite adsorbent.
For example, K.sub.MX.sup.PX denotes a selectivity of a zeolite adsorbent for para-xylene over meta-xylene and K.sub.PX.sup.DS denotes a selectivity of a zeolite adsorbent for a desorbent over para-xylene.
2. Description of the Prior Art
C.sub.8 aromatic isomers are supplied from, for example, naphtha-crackers, reformers or disproportionation process of toluene. Conventionally C.sub.8 aromatic isomers are separated by distillation, crystallization, extraction of HF-BF.sub.3 complex of MX or selective adsorption by a zeolite adsorbent. Also, separation of C.sub.8 aromatic isomers is conducted on an industrial scale with an isomerization process which is to convert, MX, OX, and EB into PX. Of many industrial separation processes the selective adsorption process by a zeolite adsorbent is generally considered the most economical process.
In conducting the selective adsorption of C.sub.8 aromatic isomers by a zeolite adsorbent the important points are firstly to select a zeolite adsorbent having a high selectivity for one C.sub.8 aromatic isomer over another C.sub.8 aromatic isomer and secondly to select a most appropriate desorbent for the separation process employed.
It is known that aromatic hydrocarbons such as toluene, benzene, diethylbenzene and cumene; naphthalenes; alcohols; and ketones can be used as desorbents in the adsorption separation of C.sub.8 aromatic isomers.
In conducting the adsorption separation process there are two types of separation systems. One is a so-called "displacement chromatography" and the other a so-called "elution chromatography". According to the so-called "displacement chromatography", an C.sub.8 aromatic isomer adsorption band migrates while a frontal and a rear desorbents are kept adjacent to both frontal and rear boundaries of the C.sub.8 aromatic isomer adsorption band, respectively, and the C.sub.8 aromatic isomer adsorption band and the desorbent adsorption band are kept separated from each other. According to this type of operation, the boundaries between the C.sub.8 aromatic isomer adsorption band and the desorbent do not diffuse and, therefore, advantageously products can be obtained at a high concentration of C.sub.8 aromatic isomers. However, there is required a regeneration operation of displacing the rear desorbent by the frontal desorbent after the development of the C.sub.8 aromatic isomer adsorption band and thus a large amount of heat consumption is indispensable for heating or washing with a large amount of the frontal desorbent.
On the other hand, according to the so-called "elution chromatography", a desorbent enters the C.sub.8 aromatic isomer adsorption band and accordingly, when the selectivity for one C.sub.8 aromatic isomer over another C.sub.8 aromatic isomer is low and a considerably long migration distance of the C.sub.8 aromatic isomer adsorption band is necessary for achieving a desired degree of separation, the C.sub.8 aromatic isomer adsorption band is diluted with a large amount of the desorbent, resulting in a large heat consumption.
According to many of the conventionally proposed adsorption separation processes by a zeolite adsorbent, PX is, in general, recovered as a product. One separation process comprises adsorbing PX alone as an adsorptive component on a zeolite adsorbent in an adsorption separation zone to recover PX as a product and passing OX, MX and EB as non-adsorptive components into an isomerization reaction. Another separation process comprises adsorbing PX alone on a zeolite adsorbent in an adsorption separation zone to recover PX as a product and passing OX and MX into an isomerization reaction zone after removal of EB from OX, MX and EB by super rectification. A further separation process as described in Japanese Patent Publication Nos. 29300/1977 and 29730/1977 comprises passing a feed mixture and an isomerization effluent to an OX separation zone to separate OX by distillation, selectively retaining PX and Eb as adsorptive components on a zeolite adsorbent in a first adsorption separation zone, passing MX as a non-retained component into an isomerization reaction zone while selectively retaining PX or EB on a zeolite adsorbent of a second adsorption separation zone to separate PX and EB. In the first and the second processes of the above described three methods, the selectivities which determine the ease in separating OX, MX and EB as non-absorptive components from PX as an adsorptive component are K.sub.EB.sup.PX and K.sub.MX(OX).sup.PX and the degree of separation is determined by the lowest value among these selectivities. In the third process as described above, the selectivity which determines the degree of separation in the first adsorption separation zone is K.sub.MX.sup.EB and K.sub.MX.sup.PX and the selectivity which determines the degree of separation in the second adsorption separation zone is K.sub.EB.sup.PX. According these conventional processes, there are such restrictions that the separation operation must be conducted by a zeolite adsorbent whose K.sub.MX(OX).sup.PX is nearly equal to the low K.sub.EB.sup.PX and under the conditions where K.sub.MX.sup.PX is nearly equal to the low K.sub.EB.sup.PX and K.sub.MX(OX).sup.EB. In order to obtain a desired degree of separation in conducting the adsorption separation with a low selectivity, a long migration distance of an C.sub.8 aromatic isomer adsorption zone is required, and the C.sub.8 aromatic isomer adsorption zone is diluted with a large amount of a desorbent and the process operation becomes a so-called "elution chromatography". The desorbent used in this conventional process can be selected within a wide range of desorbents but from the economical viewpoint the process is not advantageous.